1. Field of Invention
This invention is an economical method for pretreating lignocellulosic biomass for the production of liquid and gaseous biofuels. “Biomass,” as that term is used herein, includes any plant matter, plant residual, or waste substrate containing lignin, cellulose or hemicellulose.
2. Background Art
Plant cell walls are comprised of cellulose, hemicellulose and lignin, and collectively these compounds are called “lignocellulose.” The production of biofuels from lignocellulosic biomass follows the pathway of 1) size reduction, 2) pretreatment to increase susceptibility to hydrolytic enzymes, 3) hydrolysis and fermentation, 4) extraction and recovery. The first step in biofuel production after size reduction is pretreatment wherein the biomass fibers are broken apart from the lignin structure to expose the cellulose and hemicellulose, destroy the cellulose crystalline structure and thereby produce degradable amorphous cellulose and hemicellulose. All lignocellulose pretreatments can be divided into four main categories:    1) Physical methods, including extrusion, dry milling (chipping, ball milling, and grinding), wet milling, irradiation, microwave, and swelling reagents (e.g., ZnCl2);    2) Chemical methods, including dilute acids (dilute H2SO4, H3PO4, HCl, acetic acid, formic acid/HCl), alkalis (NaOH, lime, ammonia, amine), organosolv, oxidizing agents (O3, NO, H2O2, NaClO2), cellulose solvents (cadoxen), DMAc/LiCl, and concentrated H2SO4;    3) Physiochemical methods, including steam explosion with or without catalysts, CO2 explosion, ammonia fiber explosion or expansion (AFEX), hot water with flow-through, supercritical fluid extractions (CO2, CO2/H2O, CO2/SO2, NH3, H2O); and    4) Biological methods such as white rot fungi.
A number of hybrid systems have been proposed utilizing two or more of the categories described above.
The next step following pretreatment is hydrolysis through the use of enzymes, and/or hydrolytic organisms such as Saccharomyces cerevisiae or Z. mobilis to convert the cellulose and hemicellulose to sugars that can be fermented to ethanol. Pretreatment processes normally improve the enzymatic hydrolysis rates by several hundred percent. For example, the hydrolysis yield (Yh=Glucose in Sample/Estimated Potential Glucose) of grass without pretreatment was <20% in 48 hours compared to 80% with hot water, or dilute acid, or dilute alkali pretreatment over the same period (Kumar and Murthy 2011). As stated above, a large variety of pretreatment technologies exist. The common technologies are hot water, dilute acid, aqueous ammonia soaking, wet oxidation, sulfur dioxide steam explosion, alkali soaking, and ammonia fiber explosion.
Although significant benefits are accrued through the use of pretreatment, the cost of pretreatment represents 30% to 40% of the cost of lignocellulosic ethanol production. The recalcitrance of the lignocellulosic substrate impacts the cost of pretreatment and downstream processes involving lignocellulose conversion to fuel. Pretreatment methods are thus strongly associated with downstream costs, including the enzymatic hydrolysis rate, enzyme loading, power consumption for mixing, product concentration, detoxification if inhibitors are generated, product purification, power generation, waste treatment demands, and other process variables. Starch-based grain or corn ethanol production uses the most economical pretreatment process, namely high-pressure hot water (pressure cooker) pretreatment. Other technologies for lignocellulosic pretreatment are more expensive since chemicals such as ammonia, concentrated or dilute acids, caustics such as lime or sodium hydroxide must be purchased. If acids are used, the treated biomass must be detoxified through a separate treatment step.
One of the most effective pretreatment technologies is ammonia fiber explosion (AFEX) where liquid ammonia and water are added to the dry biomass substrate under pressure, heated in a pressure reactor for a short duration of time (5 minutes±), and exploded through a rapid release of pressure resulting in the vaporization of the ammonia and the disintegration of the fibers (U.S. Pat. Nos. 3,707,436; 4,600,590; 5,171,592). Ninety to 95% of the ammonium can be recovered through a variety of subsequent process steps. Many studies and U.S. patents have focused on Ammonia Fiber Explosion since it is one of the most promising pretreatment techniques that least affect downstream cost and performance. Its use has been analyzed for the pretreatment of a variety of crop residues, grains, and both soft and hard woods. The primary disadvantage of the process, however, has been the high capital and operating cost associated with capturing and recycling ammonia. A brief history of its evolution follows:
In 1967, James J. O'Connor culminated his research on a new method for producing wood pulp that included the steps of impregnating a mass of lignocellulose chips with anhydrous ammonia, heating without the use of added steam within a closed reactor under pressure, and then suddenly releasing the pressure to cause the explosive removal of ammonia and the deformation and disintegration of the wood chips to a fibrous condition in which “the fibers were flexible, kinked, twisted, and curled.” O'Connor had observed that “nitrogenous agents such as ammonia and amine-nitrogen compounds that have an —NH2 group, for example, effectively swell and plasticize wood (O'Connor 1971). Twenty years later, Dale applied for and received a patent on “a method for increasing the reactivity and digestibility of cellulose with ammonia” (Dale 1986). The method was described as follows: “The cellulose is contacted, in a pressure vessel, with a volatile liquid swelling agent having a vapor pressure greater than atmospheric at ambient temperatures, such as ammonia. The contact is maintained for a sufficient time to enable said agent to swell the cellulose fibers. The pressure is rapidly reduced to atmospheric, allowing said agent to boil and explode the cellulose fiber structure. The rapid pressure reduction also causes some freezing of the cellulose. The agent is separated from said cellulose and recovered for recycling.” The process was subsequently referred to as “Ammonia Freeze Explosion.”
In 1987, Norman published a paper on the transformations of organic matter solubilized by anhydrous ammonia (Norman et al., 1987). Norman had observed that anhydrous ammonia solubilized the majority of the organic matter in soil and made such organic matter available for bacterial consumption and the subsequent release as carbon dioxide.
A summary of other patents and patent applications that followed are presented below.                Patents issued to Dale, U.S. Pat. Nos. 4,600,590 and 5,037,663, describe the use of various volatile chemical agents to treat the cellulose containing materials, particularly ammonia by what came to be known as the AFEX process (ammonia freeze or ammonia fiber explosion).        U.S. Pat. No. 5,171,592, issued to Holtzapple et al. (1992), provides an AFEX process in which the biomass is treated with liquid ammonia or any other appropriate swelling agent, exploded, and the swelling agent and the treated biomass are recovered.        U.S. Pat. No. 5,366,558 issued to Brink disclosed a process that uses two stages to hydrolyze the hemicellulose sugars and the cellulosic sugars in a countercurrent reactor.        U.S. Pat. No. 5,188,673 employs concentrated acid hydrolysis, which has the benefit of high conversion of biomass, but suffers from low product yields due to degradation and the requirement of acid recovery and recycle. Sulphuric acid concentrations used are 30-70 weight percent at temperatures less than 100° C.        U.S. Pat. No. 5,473,061 issued to Bredereck et al. (1995) describes a process which involves bringing the cellulose in contact with liquid ammonia at a pressure higher than atmospheric pressure in a pressure vessel and subsequent expansion by rapid reduction of the pressure to atmospheric pressure to activate the cellulose for subsequent chemical reactions.        U.S. patent application US 2007/0031918A1 by Dunson et al. provides a process in which the biomass at relatively high concentration is treated with relatively low concentration of ammonia relative to the dry weight of the biomass. The ammonia-treated biomass is then digested with a saccharification enzyme to produce fermentable sugars. The process utilizes vacuum for better ammonia penetration and recovery; it also uses a plasticizer for softening.        U.S. patent application US 2008/0008783A1 by Bruce Dale et al. disclosed a pretreatment process using concentrated ammonium hydroxide under pressure to improve the accessibility/digestibility of the polysaccharides from a cellulosic biomass. It also uses a combination of anhydrous ammonia and concentrated ammonium hydroxide solutions.        In December 2009, Dale applied for a U.S. patent for the separation of proteins from grasses utilizing the ammonia fiber explosion process (Dale et al., 2009). The process was described as: “A process for extracting an aqueous ammonium hydroxide solution from a plant biomass after an Ammonia Fiber Explosion (AFEX) process step is described. The proteins can be separated before or after a hydrolysis of sugar precursors (carbohydrates) from the biomass to produce sugars for fermentation to produce ethanol. The proteins are useful as animal feeds because of their amino acid food values.”        In September 2009, Zhang applied for a U.S. patent on a highbrid, dilute-acid process that provides a novel method for conversion of plant material, including material containing cellulose, hemicellulose, and lignocellulose, to usable energy sources, such as carbohydrates, ethanol, and hydrogen (Zhang 2009). In general, the invention provides a novel lignocellulosic pretreatment by use of concentrated acid and cellulose solvents.        In March 2010, Geros applied for U.S. patent for a pretreatment process that involved sequestering of ammonia nitrogen following fermentation through the addition of acid (Geros 2010).        In September 2010, Kreisler applied for a US patent on a complex process for conditioning biomass using the steps of 1) flash dessicating the biomass to reduce a particle size of the biomass; 2) mixing the biomass with a liquid carrier; and 3) exposing the biomass and the liquid carrier to a mechanical hydrodynamic cavitation process (Kreisler 2010).        In October 2010, Parekh submitted a patent application for a process utilizing microorganisms, such as a Clostridium strain, in several stages to produce a final, fermented end product (Parekh 2010).        In December 2010, Sudhakaran applied for a US patent on a multistep process and system for the separation of biomass components into individual components such as cellulose, hemicellulose and lignin. The invention provides a process for separating lignin in its native form (Sudhakaran and Samuel 2010).        
Over the past several years, an equally significant number of publications dealing with biomass pretreatment have been published in a variety of journals and books. In 2005, Mosier published a review describing the features of biomass pretreatment technologies (Mosier et al. 2005). Dale recently published research findings on the parameters controlling ammonia fiber explosion for the enzymatic hydrolysis of corn stover (Teymouri et al. 2005.). Sousa, et al., recently published an informative review and assessment of lignocellulosic pretreatment technologies (Sousa et al. 2009).
In spite of a number of investigations to optimize the Ammonia Fiber Explosion process it remains a complex process that requires considerable energy to heat and pressurize the water, ammonia, and biomass. The process is complicated by the ammonia recovery step. Ammonia provides two important advantages for use in this process. First, ammonia reacts with and solubilizes the organic substrate. Second, ammonia is volatile at low temperatures and thus can be easily expelled from the pretreated biomass in a gaseous state and subsequently recovered through a complex process.
E3 Recent Research
Over the years the present inventor, doing business as E3 Environmental Energy & Engineering Co., has conducted extensive research into high solids anaerobic digestion of a variety of substrates and the removal and recovery of ammonia nitrogen from anaerobic digestate, where the term “high solids” is here defined to mean a solids influent composition comprising at least 15 percent solids (w/v). He recently received U.S. Pat. No. 7,806,957 for his process for anaerobic digestion of high solids to produce balanced fertilizer and U.S. Pat. No. 7,811,455 for his method for recovery of ammonia nitrogen therefrom. He recently demonstrated the profitable removal and recovery of ammonia from liquid anaerobic digestate. The process recovered ammonium bicarbonate and carbonate as a liquid or solid product from the stripped ammonia while producing biomethane gas. The process used no chemicals and very little power to recover the ammonia. The recovered ammonia was then combined with CO2 from the anaerobic digester's biogas, thereby producing biomethane fuel and ammonium bicarbonate/carbonate (U.S. patent application Ser. No. 13/373,860).
During the ammonia recovery research described above, he investigated the kinetics of ammonium bicarbonate solids precipitation with biogas CO2 and stripped ammonia condensate. The experiments were carried out using various concentrations of liquid ammonium bicarbonate condensate obtained through the digestate ammonia stripping process. The ammonia condensate (NH4+H2O) was injected into 1 L Tedlar bags filled with CO2. The condensate collapsed the bags while forming ammonium bicarbonate solids in an exothermic reaction. During a side experiment, the collapsed bags containing the ammonium bicarbonate solids were briefly (seconds) placed in a microwave, vaporizing the solids and refilling the bag with CO2, NH3, and H2O. Upon removal of the bags from the microwave, the bags collapsed while the ammonium bicarbonate returned to the solid form. A small amount of external energy caused work to be performed through gas expansion (PV), and the removal of the bag to ambient temperature conditions resulted in an exothermic reaction, releasing energy and forming an ammonium bicarbonate precipitate from the CO2, NH3, and H2O in the bag. The cycle was repeated many times.